Seabed excavation is often performed by dredging, for example to retrieve valuable alluvial placer deposits or to keep waterways navigable. Suction dredging involves positioning a gathering end of a pipe or tube close to the seabed material to be excavated, and using a surface pump to generate a negative differential pressure to suck water and nearby mobile seafloor sediment up the pipe. Cutter suction dredging further provides a cutter head at or near the suction inlet to release compacted soils, gravels or even hard rock, to be sucked up the tube. Large cutter suction dredges can apply tens of thousands of kilowatts of cutting power. Other seabed dredging techniques include auger suction, jet lift, air lift and bucket dredging.
Most dredging equipment typically operates only to depths of tens of meters, with even very large dredges having maximum dredging depths of little more than one hundred meters. Dredging is thus usually limited to relatively shallow water.
Subsea boreholes such as oil wells can operate in deeper water of up to several thousand meters depth. However, subsea borehole mining technology does not enable seafloor mining.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.